refactor(notes): a snippet's and lesson's stored title is its name; the trigger joins it only in the embedded document (milestone 427)

The title was `subject — trigger` because the stored title WAS the
embedded one, and the join is what makes these kinds rank on the
situation they apply to (#2485). Every surface that shows a title then
showed the trigger too -- menus, lists and search rows ran to kilobytes.

- embeddings.document_title(title, note_type, data, body) joins the
  trigger from `data` (body fallback) at embed time. Idempotent: an
  un-migrated composed title comes out the same, never doubled. The
  embed path, the startup backfill and the dedup gate's semantic signal
  all use it, so the embedded text -- and every vector -- is unchanged.
- Writers store the subject: snippet create/update (service, REST, MCP)
  and lesson_document. Both compose_title helpers are removed.
- Readers: dedup takes `data`; the menus strip the embedded title from a
  passage; list rows project `when_to_use`, which SnippetListView reads.
- 0108 rewrites existing rows on an exact `' — ' || <own trigger>`
  suffix with raw SQL, leaving updated_at alone so the backfill does not
  re-embed the corpus for identical vectors. Downgrade recomposes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/api/snippets.ts

@@ -91,6 +91,9 @@ export interface SnippetListItem {
    *  hit can be flagged as being in a DIFFERENT language than the file being
    *  written, which is a shape to adapt rather than code to paste. */
   language?: string;
+  /** When to reach for it, projected from the `data` mirror. The title is the
+   *  name alone since milestone 427, so this is where the situation lives. */
+  when_to_use?: string;
   /** Always present from the backend, zero-filled for records with no events. */
   usage?: SnippetUsage;
   /** Present on the detail record; the list feed carries it when a check has